Devices for sensing the speed of a rotating member are well known (see, e.g., U.S. Pat. No. 4,970,463 which describes Hall-effect sensors used to detect the teeth or notches on rotating ferrous wheels, e.g., gears). In such devices, the length of the airgap (the spacing between the sensor and the passing ferrous teeth) must be sufficiently small to assure reliable detection of each passing tooth. The precise length of the airgap may be difficult to establish when, for example, there is a large tolerance stackup or when interchangeability of sensors is a requirement. Moreover, in many installations the airgap is blind or hidden and hence it may be difficult, expensive, and impractical to precisely measure.